Barlow was perhaps one of the last great amateurs in science. When he was in his early thirties, after he attained the leisure afforded by an inheritance from his father, he began to study crystallography, which became his life's work. Barlow was elected to the Royal Society in 1908 and was president of the Mineralogical Society in 1915-1918, and his career spanned a critical stage in the development of crystallography.

Barlow's 1883 paper started with the same premise first proposed by Johannes Kepler-that a crystal may be regarded as resembling an orderly stack of cannonballs. By treating the atoms as individual spheres, Barlow was able to stack them in five arrangements that he considered to be the only "very symmetrical" ones; these correspond to structures known today as simple cubic, body-centered cubic, face-centered cubic, hexagonal closest-packed, and one kind of double hexagonal structure.

In this first paper, however, Barlow also recognized that body-centered cubic and simple cubic structures admit packing of spheres of two kinds. Barlow knew that many two-element crystals, such as sodium chloride and related alkali halides, form cubic crystals. From this fact, Barlow suggested that the alkali halides adopted structures in which the two kinds of atoms were arranged either in the body-centered or simple cubic structures. In 1913, the Braggs carried out the first structure determination by X-ray diffraction; the materials they chose to study - alkali halides - had exactly the structures Barlow had suggested. Barlow's educated guesses were sometimes off the mark, however, and his proposed structures for many other substances, such as ice, quartz, and calcium carbonate, were eventually proven to be wrong.

One of Barlow's other great achievements was his independent derivation of the number of space groups, a discovery also made in 1890 by E. S. Fedorov and Arthur Schoenflies. Unfortunately, Barlow did not publish his own derivation of the 230 space groups until 1894, and consequently he generally received little credit for this achievement. In contrast with Fedorov and Schoenflies, however, whose work was purely mathematical, Barlow had as his goal nothing less than a total explanation of crystals in terms of structure.